Ionic liquids (ILs) are organic salts with a low melting point and very diverse molecular composition and structure as well as potential applications in several fields. Their structure is consistently ionic and it involves an organic cation and a counter ion that may be organic or inorganic. Initially, these compounds were considered as “green solvents”, due to their especial physicochemical properties, for example low vapor pressure, structural stability, etc. The ILs spurred expectations of great interest and have generated more than 6,000 scientific articles published in the last 10 years. Since then, the topic evolved to more sophisticated expectations in the chemical industry and materials engineering applications, such as in the field of catalysis, organometallic chemistry, C—C coupling, etc. Imidazolium like derivatives having distinct alkyl chains attached to the ring-nitrogen atoms are being studied profusely in both symmetric and non-symmetric structures. In addition, the positive charge associated to the organic nucleus imposes its association with a counter ion, which gives a different character as well as several physicochemical properties. Among the most common ILs are the alkyl-trifluoromethanesulfonate amide, bis(trifluoromethylsulfonyl) dicyanamide, hexafluorophosphates, tetrafluoroborates, acetates hydroxides and halides.
The most common method for the synthesis of these ionic liquids is the direct combination of the halide salt with a metal halide, which is used for the synthesis of ionic liquids such as halogenoaluminate (III) and chlorocuprate (I). The latter is particularly sensitive to oxygen which makes it that its use in organic synthesis is rather limited.
Some reports on the synthesis of ionic liquids exhibit different synthesis routes, especially the modifications based on the Radziszewski type reaction (U.S. Pat. No. 5,077,414 Arduengo et al.), who reported the use of reactive α-di-calbonil type, preferably aldehydes in solution, or paraformaldehyde, s-trioxane and/or polyoxymethylene, a primary amine and an acid (preferably with pKa below 2, for example, hydrochloric, sulfuric, etc.), with yields above 98%, reaction times near 0.5 to 24 h and reaction temperatures ranging from −10° C. to 200° C.; likewise, it has been reported (Organic Process Research & Development 2010, 14, 1102-1109) the synthesis of ionic liquids using n-butyl amine as raw material, glyoxal, formaldehyde and tetrafluoroboronic acid, acetic acid and hydrogen chloride, with reaction times ranging from 2 to 20 h, at 10 to 20° C., with yields of less than 59%.
In general, ILs have a hetero-substitution (i.e., 1-alkyl-3-methylimidazolium, where the alkyl group is regularly a group of low molecular weight).
The influence of this hetero-substitution has been the focus of many studies, which indicate that the cations allow the salts to have a low melting point and low viscosity (2). Furthermore, the salts of the hetero-substitution like 1,3-di-alkylimidazolium are not selectively obtained by using two different amines in the modified Radziszewski type reaction, which gives a mixture of compounds (3). In this sense, there are few reports about the physicochemical properties (4) and the potential application of homo-substituted imidazolium type salts.
The literature (4) mentioned that derivatives of N-propylamine have a lower viscosity than derivatives from N-butylamine, regardless of the counter ion chosen. Another interesting fact of these homo-substituted derivatives is their thermal stability, which is independent of the chain size and cation type but it seems to diminish with the anion nucleophilicity, which is presumed to involve a nucleophilic attack on the alkyl substituent, an initiator of decomposition (5).
Recently, the functionalization of ionic liquids (FILs), also called ionic liquids for specific tasks (TSILs “task-specific ionic liquids”), covalently incorporate functionality that gives a different reactivity, i.e., ILs with amines, amides, nitriles, ethers, alcohols, acids, ureas and thioureas, which have been designed and synthesized in this way were applied in catalysis, organic synthesis, gas adsorption, analytical chemistry and preparation of new materials (6).
In addition, polyethylene glycols (PEG) are polymers formed from polymerization of oxirane derivatives. These compounds have received more attention from the environmental point of view because they are inexpensive, non-volatile and easily degradable (7). The high reactivity of the oxirane group basis allows to carry out the preparation of compounds with industrial interest, based on alcohols, amines and carboxylic acids, with potential applications in different areas, i.e., IL's-PEG derivatives have been used as solvents in Heck-type coupling reactions (8), electrolytes for energy conversion and energy storage devices, solar cells and super-capacitors (9).
The current trend in the use of LI's involves raw materials that are known for its synthesis purity, which is a major concern in this field. In addition, a growing number of low melting point ILs have not been applied in its liquid state, thus offering new opportunities and discoveries. There is no doubt about the interest in the search and use of new ILs as green solvents, reagents, catalysts and feedstock for the synthesis of new materials. In parallel, there is a marked trend towards the search of new routes for the synthesis of IL's with the purpose of obtaining technical and economic advantages.
In this context, an object of the present invention is to provide a process for the synthesis of ionic liquids based on the imidazolium ring type, which comprises the reaction of primary amines with aldehydes of different type and a mineral inorganic acid, which is carried out by cyclo-condensation type reactions, this is Radziszewski's method, which involves the reaction with oxirane derivatives of di-polymerized ILs to produce a certain degree of polymerization from 2 to more than 50.